Concrete screed movement against extension concrete floor

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26 May 2022
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Hello,

We are currently redecorating and have taken up the carpet ready for screeding and LVT.
Once we got the carpet up we noticed that there was a crack right where the original floor meets the extension floor. When you stand on the old floor, there is movment of a few millimetres up and down. Which flooring contractors have said is no good for screeding over.

The original floor is an old concrete screed I believe, which is approx 20mm thick which I am told has a bitumen DPM painted underneath and then a thin layer of bitumen ontop which was to seal the thermoplastic tiles down on top.

First question is, how do we stop the movement of the old concrete screed against the new extension floor? As if we just screed over the crack I presume the new screed will just crack with the movement?

Second question is, as we have had a few damp problems I was planning on getting a new liquid DPM such as Ardex DPM 1c. I have read a few posts which suggest a screed of Ardex NA, a layer of Ardex 1c and then a layer of Ardex NA which the LVT can then be lay onto. Can this be done straight over the thermoplastic tiles and the bitumen?

Lastly, a flooring contractor pulled a small amount of the old concrete screed up to see why there was movement so there is a patch that will need filling. Can it just be filled with Ardex A46 concrete filler? I had wondered if it would be a good idea to pull up the edge of the old screed by about 150mm all the way along the crack and fill with A46 and try to push some under the old concrete screed to try and stop the movement. Don't know if this is a good idea or not.

Thanks
 

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